In general, a document handling apparatus such as a paper sheet determination apparatus includes a plurality of detecting means for detecting a type, authenticity or a defacement degree of a paper sheet such as valuable securities. The determination of a type or a defacement degree of paper sheet is usually performed based on recognition of the most characteristic visible image. That is because the paper sheet is fundamentally designed and have colors, characters, numbers and others drawn thereon on the assumption that humans can easily distinguish them, and a paper sheet determination apparatus uses an image sensor, a color determination sensor and others in particular.
When a human determines a defacement degree of a paper sheet, two methods are usually adopt. One is a method of detecting flexibility or toughness of medium. This is a method that uses a feeling of fingers to detect that a paper sheet loses toughness when fibers of a paper sheet as a material are fractured or a liquid adheres while the medium is circulated for a long time. Another one is a method of visually detecting contamination on a surface of a paper sheet. Empirically, contamination, wrinkles, folds and others of a non-printed portion of a paper sheet, e.g., a watermark part or a part having a relatively low printing ink concentration in case of valuable securities are observed to determine a defacement degree.
Even the paper sheet determination apparatus adopts a technique of determining a defacement degree of a paper sheet based on the above-described two methods. According to the former method, toughness is determined based on an intensity level of sound produced when a measurement target medium is carried. On the other hand, the latter method detects brightness of a non-printed part paper sheet, rugosity of the non-printed part, a transmission factor of the non-printed part, shading characteristics of a folded part of a paper sheet, brightness of a periphery of the paper sheet, brightness of the entire paper sheet, shading characteristics of the entire paper sheet and others.
As described above, in the paper sheet determination apparatus that determines, e.g., a defacement degree of a paper sheet by using an image, the defacement degree is detected based on shading or wrinkles of a non-printed part which is not affected by printing. Therefore, accurately determining a paper sheet which is contaminated in parts other than a printed part or a paper sheet which is less contaminated but not officially sealed is difficult. Further, since a control unit in the paper sheet determination apparatus evaluates brightness of a specific region or a characteristic amount of a variance value by itself, a determination accuracy is low, and results different from the human sense are often obtained.
On the other hand, according to the method of detecting flexibility or toughness of a paper sheet, an accidentally contaminated paper sheet, e.g., a paper sheet having stain of coffee or an officially sealed note having wrinkles from washing cannot be accurately determined, and a defacement degree cannot be highly accurately determined in any case.